Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame that defines manifolds for directing flow of reducing, oxidizing, coolant and product fluids. Each individual cell generally includes an anode electrode or catalyst and a cathode electrode or catalyst separated by an electrolyte. A reducing fluid such as hydrogen is supplied to the anode catalyst, and an oxidant such as oxygen or air is supplied to the cathode catalyst. In a cell utilizing a proton exchange membrane as the electrolyte, the hydrogen electrochemically reacts at a surface of the anode catalyst to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode catalyst, while the hydrogen ions transfer through the electrolyte to the cathode catalyst, where they react with the oxidant and electrons to produce water and release thermal energy. It is common in fuel cell technology to refer to the locations of the aforesaid electrochemical reactions as "electrodes", which often is meant to include both a catalyst such as platinum and a support structure such as a porous carbon substrate. However, occasionally the term "electrode" also includes portions of the support structure that does not include a catalyst, such as an edge portion. For purposes of clarity herein, the term "catalyst" will be used, as "anode catalyst" and "cathode catalyst", instead of "electrode" to identify only the location of catalysts that catalyze electrochemical reactions within a fuel cell.
The anode and cathode catalysts of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is a proton exchange membrane ("PEM") electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, nonconductive matrix between the anode and cathode catalysts.
It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention. Furthermore, because specific electrode or catalyst reactions proceed rapidly in a PEM fuel cell, high power densities can be obtained at low catalyst loadings leading to a low cost power plant with high power densities. Finally, because a PEM fuel cell operates at low temperatures, a power plant including a stack of such fuel cells can be started rapidly providing operational flexibility for transportation and stationery applications. As is well-known however, PEM cells have significant limitations especially related to liquid water transport to, through and away from the PEM, and related to simultaneous transport of gaseous reducing and oxidant fluids to and from the catalysts adjacent opposed surfaces of the PEM. The prior art includes many efforts to minimize the effect of those limitations.
In operation of a fuel cell employing a PEM, the membrane is saturated with water, and the anode catalyst adjacent the membrane must remain wet. As hydrogen ions produced at the anode catalyst transfer through the electrolyte, they drag water molecules with them from the anode to the cathode. Water also transfers back to the anode from the cathode by osmosis. Product water formed at the cathode catalyst is removed by evaporation or entrainment into a circulating gaseous stream of oxidant, or by capillary action into and through a porous support fluid transport layer adjacent the cathode. Porous water transport plates supply liquid water from a supply of coolant water to the anode catalyst and remove water from the cathode catalyst returning it back to the coolant water supply, and the plates thereby also serve to remove heat from the electrolyte and catalysts, as described in U.S. Pat. Nos. 4,769,297 and 5,503,944 assigned to the assignee of the present invention.
During operation of PEM fuel cells, it is critical that a proper water balance be maintained between a rate at which water is produced at the cathode catalyst and rates at which water is removed from the cathode catalyst and at which liquid water is supplied to the anode catalyst. An operational limit on performance of a fuel cell is defined by an ability of the cell to maintain the water balance as electrical current drawn from the cell into the external load circuit varies and as an operating environment of the cell varies. For PEM fuel cells, if insufficient water is returned to the anode catalyst, adjacent portions of the PEM electrolyte dry out thereby decreasing the rate at which hydrogen ions may be transferred through the PEM and also resulting in cross-over of the reducing fluid leading to local over heating. Similarly, if insufficient water is removed from the cathode, the cathode catalyst may become flooded effectively limiting oxidant supply to the cathode and hence decreasing current flow. Additionally, if too much water is removed from the cathode by the gaseous stream of oxidant, the cathode may dry out limiting ability of hydrogen ions to pass through the PEM, thus decreasing cell performance.
As fuel cells have been integrated into power plants developed to power transportation vehicles such as automobiles, trucks, buses, etc., maintaining an efficient water balance within the power plant has become a greater challenge because of a variety of factors. For example, with a stationary fuel cell power plant, water lost from the plant may be replaced by water supplied to the plant from off-plant sources. With a transportation vehicle, however, to minimize weight and space requirements of a fuel cell power plant the plant must be self-sufficient in water to be viable. Self-sufficiency in water means that enough water must be retained within the plant to offset losses from reactant fluids exiting the plant in order to efficiently operate the plant. Any water exiting the plant through a plant exhaust stream consisting of a cathode exhaust stream of gaseous oxidant and/or an anode exhaust stream of gaseous reducing fluid must be balanced by water produced electrochemically at the cathode catalyst and water retained within the plant.
For example, an ambient pressure, gasoline powered PEM fuel cell must be self-sufficient in water to be a viable power source for vehicles. Such a power source requires fuel processing components to process the gasoline into a hydrogen rich reactant fluid. The fuel processing components use water heated to steam in a boiler to aid in processing the gasoline, and the water for the fuel processing components must be supplied from water produced at the cathode in the fuel cell as a result of the above described electrochemical reaction. As is well-known however, the water produced at the cathode catalyst is swept from the cell within the cathode exhaust stream. It is known to recover some of the water in the cathode exhaust stream by passing the cathode exhaust stream through a condensing heat exchanger to cool the stream and thereby condense the water out of the stream. The condensed water is then accumulated and directed to the fuel processing components as required to maintain the plant in water balance.
An example of a PEM fuel cell power plant using a condensing heat exchanger is shown in U.S. Pat. No. 5,573,866 that issued on Nov. 12, 1996 to Van Dine et al., and is assigned to the assignee of the present invention, and which patent is hereby incorporated herein by reference. Many other fuel cell power plants that use one or more condensing heat exchangers are well-known in the art, and they typically use ambient air streams as a cooling fluid passing through the exchanger to cool the plant exhaust streams. In Van Dine et al., the heat exchanger is used to cool an exhaust stream exiting a cathode chamber housing the cathode catalyst. Prior to entering the cathode housing, the same stream provides air as the oxidant for the cathode catalyst, and upon leaving the chamber the stream includes evaporated product water and some portion of methanol, the reducing fluid, that has passed through the PEM. The condensing heat exchanger passes the cathode exhaust stream in heat exchange relationship with a stream of cooling ambient air, and then directs condensed methanol and water indirectly through a piping system back to an anode side of the cell.
While condensing heat exchangers have enhanced water balance and energy efficiency of ambient fuel cell power plants, the heat exchangers encounter decreasing water recovery efficiency as ambient temperatures increase. Where the power plant is to power a transportation vehicle such as an automobile, the plant will be exposed to an extremely wide range of ambient temperatures. For example where an ambient air cooling fluid passes through a heat exchanger, performance of the exchanger will vary as a direct function of the temperature of the ambient air because decreasing amounts of liquid precipitate out of power plant exhaust streams as the ambient air temperature increases.
An additional complication of known fuel cell power plants designed for use in transportation vehicles is also related to fluctuations in ambient air conditions. Fuel cells of such plants typically utilize ambient air as the oxidant directed to the cathode catalyst. Hot and dry ambient air increases a risk that the cathode catalyst will dry out because such hot, dry air removes water more quickly by evaporation than does cool, moist oxidant supply air. Such hot, dry ambient air raises a dewpoint of the plant exhaust stream effectively moving the plant out of water balance.
Consequently, many efforts have been undertaken to prevent excess water loss resulting in drying out of the cathode catalyst and adjacent electrolyte especially in PEM fuel cells, including: directing liquid condensate from condensing heat exchangers to humidify gaseous reactant and oxidant streams entering the cell; adding porous support layers and water transport plates in fluid communication with the catalysts for movement of coolant water through adjacent cells; generating a pressure differential on the anode side of the cell wherein gaseous reducing fluids are maintained at a slightly higher pressure than coolant water and anode supply water passing through the porous support layers adjacent reducing gas distribution channels so that the pressure differential assists water transport through the porous support layers and cell; and, increasing air utilization by the cathode through decreasing volumetric flow of the oxidant stream by the cathode. Such efforts at maintaining efficient water balance involve additional cost, weight, volume burdens, fuel cell performance penalties, and often require complicated control apparatus.
An alternative approach to enhancing water balance for fuel cell power plants in transportation vehicles is to pressurize the cell and related components. This increases reactant concentrations in high pressure gaseous streams and also reduces water loss through plant exhaust streams by reducing volumetric flow of the streams. Such pressurized fuel cell power plants, however, require additional cost, weight and control apparatus in providing appropriate pressure housings and controls, and pressurized plants require additional energy derived from the plant to operate pressurizing pumps, valves, fans, etc., and are not known to be practical for portable power plants.
Accordingly, known pressurized plants and plants that employ ambient air as the cathode oxidant or that use ambient air for condensing heat exchangers are incapable of maximizing an efficient water balance and minimizing operating energy requirements because of their above described characteristics. It is therefore highly desirable to produce a fuel cell power plant that minimizes reliance upon ambient air cooled condensing heat exchangers to maintain the plant in water balance.